The present invention relates to a filler for use in paper manufacture as defined in the preamble of claim 1. Moreover, the invention relates to a procedure for producing said filler.
In the present application, xe2x80x98paperxe2x80x99 refers to various kinds of paper and cardboard, manufactured with paper and cardboard machines, coated or uncoated.
Today, the direction of development of paper products is to an increasing degree determined by customers and legislative measures. The buyers of printing paper want to reduce the postage expenses and the amount of waste produced. Further, packages are subject to waste processing charges dependent on weight. Generally, it seems that energy taxes and environmental protection taxes are being imposed on the price of paper products as a permanent extra encumbrance. For these reasons, paper buyers want products which have a lower grammage while still meeting high quality standards.
Specification FI 931584 presents a composite product based on chemical pulp fibre or mechanical pulp fibre, with calcium carbonate crystals precipitated onto the surface of the product. Further, specification FI 944355 presents a precipitated calcium carbonate, which is in the form of calcite particle aggregates where at least 25% of the particles are of a prismatic shape. Precipitation is performed using a seed material. Further, specification EP 0604095 presents a procedure for the processing of waste material whereby calcium carbonate is precipitated onto the surface of waste material containing inorganic matter; the waste material may contain organic waste fibre, e.g. waste fibre contained in the effluent of a paper mill, with fibre length below 75 xcexcm. The calcium carbonate products described in the specifications referred to are intended to be used as fillers in paper manufacture.
In the manufacture of high-quality paper, the aim is to produce the paper with a minimum amount of raw material. When the grammage of the paper is reduced, its opacity becomes a critical factor. The opacity can be increased by increasing the filler content of the paper, which, however, generally reduces its strength. Therefore, the aim is to alter the structure of the paper while at the same time preserving the important good product qualities. For paper based communication to remain competitive in relation to electric communication, the printing quality of paper products has to be further improved.xe2x80x94These general development trends impose very high requirements on the raw materials and manufacturing processes used in paper production. To meet the requirements, very intensive efforts have been made in recent times to develop paper raw materials and manufacturing processes.
The object of the present invention is to produce a new kind of calcium carbonate based filler for paper manufacture that meets the criteria described above.
A further object of the invention is to produce a new kind of calcium carbonate based filler which has better optic properties than earlier calcium carbonate based fillers.
A further object of the invention is to produce a new kind of calcium carbonate based filler which gives the paper better strength properties, especially a better tensile strength, than earlier calcium carbonate based fillers.
A further object of the invention is to produce a new kind of calcium carbonate based filler which gives the paper a lower grammage than earlier calcium carbonate based fillers.
A further object of the invention is to produce a new kind of calcium carbonate based filler which has a higher retention than earlier calcium carbonate based fillers.
A further object of the invention is to produce a new kind of calcium carbonate based filler which reduces the overall costs of paper manufacture.
An additional object of the invention is to present a procedure for the manufacture of said filler.
As for the features characteristic of the invention, reference is made to the claims.
The invention is based, among other things, on the fact, established via corresponding investigations, that calcium carbonate can be precipitated in a way that causes it to effectively adhere to fibres and noil fibrils. The precipitation can be so performed that porous calcium carbonate aggregates held together by fibrils, i.e. fine fibres, are formed, which aggregates contain plenty of empty space and in which the calcium carbonate particles have precipitated onto the noil fibrils, adhering to them. The noil fibrils with calcium carbonate particles precipitated on them form fibres resembling pearl necklaces, and the calcium carbonate aggregates resemble clusters of pearl necklaces. The aggregates have a very large ratio of effective volume to mass as compared with the corresponding ratio of conventional calcium carbonate used as filler; effective volume here means the volume taken up by pigment in the paper.
The noil fibrils used in the filler of the invention are obtained from cellulose fibre and/or mechanical pulp fibre. The fibrils are produced from cellulose fibre and/or mechanical pulp fibre by refining. Furthermore, the noil fibrils are preferably divided into fractions, thickness 0.1-2 xcexcm, length mainly 10-400 xcexcm, suitably 10-300 xcexcm, preferably 10-150 xcexcm. Thus, the noil fibrils consist of cellulose fibre and/or mechanical pulp fibre, which means that they contain no significant amounts of inorganic matter, preferably no inorganic matter at all.
The diameter of the calcium carbonate particles in the aggregate is of the order of about 0.2-3 xcexcm, preferably about 0.3-1.5 xcexcm.
The diameter of the CaCO3 crystal aggregates is of the order of about 2-10 xcexcm.
Cellulose based noil also contains roundish noil particles which, after the precipitation process, are covered with calcium carbonate particles. In this case, as to its properties, a particle of calcium carbonate filler corresponds in the first place to a hollow filler particle having a small unit weight. In reality, the pigment is not completely hollow, because it contains noil; however, the noil has a lower unit weight than calcium carbonate, therefore the particle has a very low unit weight.
The new precipitated, calcium carbonate based filler of the invention bestows paper better optic properties and a clearly greater strength than prior-art calcium carbonate based fillers do. Furthermore, the filler of the invention allows the filler content of paper to be increased without impairing its other properties, e.g. the aforementioned strength properties, such as tensile strength. This is a significant contribution towards lowering the grammage of paper.
Further, the new filler of the invention has a clearly better retention in paper manufacture than prior-art calcium carbonate based fillers.
In consequence of the aforesaid factors, it is generally possible to achieve cost savings in paper manufacture by using the filler of the invention.
In prior art, light filler pigments are known, e.g. hollow plastic pigments, which are supposed to provide the same advantages as the calcium carbonate based filler of the present invention. However, plastic pigments are expensive, which restricts their use. When the filler of the invention is compared with pore filled or lumen filled fibre, it is to be noted that, unlike in the case of aforesaid fibres, the calcium carbonate in the filler of the invention is not-inside individual noil fibres but on the surface of the noil. In addition, the mass ratio of calcium carbonate and fibrous matter is much larger in the filler of the invention than in pore filled or lumen filled fibre. Thus, the filler of the invention is a completely new product, and it should not be confused with prior-art pore filled or lumen filled fibre.
The filler of the invention and the procedure for its manufacture differ from the specification FI 931584 referred to in the introduction especially on the basis of the thickness and length of the noil fibrils, i.e. in the present application, the noil fibrils have been refined with a pulp refiner. From the filler and manufacturing method described in specification FI 944355, the filler and manufacturing method of the present invention likewise differ in respect of the refined noil fibrils; in addition, in the specification referred to, the calcite particles are of a prismatic shape and their production requires the use of a special seed material. With respect to the product and procedure presented in specification EP 0604095, the product and procedure of the present invention likewise differ on the basis of the refinement and size of the noil fibrils; in addition, the fibre material used in the specification referred to is waste fibre containing inorganic or other matter.xe2x80x94In general, in addition to the differences stated above, the filler of the present invention differs from those described in the reference specifications on the basis of the good optic properties, strength properties and very high retention achieved, and especially on the basis of the combination of exceptionally good optic and strength properties.
The fibre used in the procedure of the invention may consist of chemical, mechanical or semi-mechancial pulp produced by any pulp or paper manufacturing method known in itself, or a combination of these in arbitrary proportions, the proportion of each component being 0-100% by weight. The refining of the pulp into noil fibrils can be implemented using any pulp refiner known in itself in pulp processing industry. When desirable, the refined noil fibrils can be screened by any fractionating method known in itself in pulp processing, e.g. using a wire screen, into the desired fibril size.
In precipitation, the noil, i.e. e.g. pulp based or other fibre based noil, is refined with a pulp refiner and screened, preferred fractions being e.g. wire screen fractions P100-P400.
In the procedure of the invention, calcium carbonate can be precipitated from any suitable solution or mixture, e.g. from a mixture of Ca(OH)2 water solution and solid Ca(OH)2 or from a calcium hydroxide water solution. Thus, precipitation can be implemented using any substance that precipitates calcium carbonate, e.g. carbon dioxide, such as gaseous carbon dioxide, suitably 1-100%, preferably 10-100% carbon dioxide gas. Instead of calcium hydroxide and carbon dioxide, it is possible to use any reaction producing calcium carbonate, e.g. the reaction between calcium chloride and sodium carbonate, producing calcium carbonate and sodium chloride.
The precipitation of calcium carbonate is performed on the surface of noil originating from cellulose fibre, suitably noil fibrils. The concentration of noil in the precipitation process is suitably 0.0001-18 w-%, preferably 0.4-10 w-%. When calcium hydroxide is used, the mass ratio of calcium hydroxide and cellulose fibres in precipitation is suitably 0.1-20, preferably 1.4-4. The precipitation temperature is in the range 5-150xc2x0 C., suitably 10-90xc2x0 C., preferably 15-80xc2x0 C.
In the carbon dioxide method, the net reaction is
Ca(OH)2+CO2⇄CaCO3+H2O.
In the chloride method, the net reaction is
CaCl2+Na2CO3⇄CaCO3+2NaCl
Calcium carbonate precipitates when the calcium compounds react according to the reaction equations. It is possible to influence the crystal size and/or shape by adjusting the reaction conditions.
The precipitation can be advantageously effected in a specific reactor where e.g. calcium hydroxide and noil are mixed. The carbonation reaction is implemented by supplying carbon dioxide, e.g. gaseous carbon dioxide, into the reactor. The progress of the reaction can be monitored by measuring the pH and conductivity of the mixture. The mixing and the supply of gas can be terminated when the pH of the mixture has fallen to the value of about 7.5, depending on the pH value of the noil. The carbonation is carried out e.g. in a water solution or mixture of Ca(OH)2.
If desirable, it is possible to add a dispersing agent, e.g. sodium hexametaphosphate (Na-HMF) or other dispersing agent(s), into the filler produced.
The filler of the invention can be used as filler as such or in any ratio of mixture (0-100%) with another filler or other fillers. The amount of filler used in paper is 0.1-50 w-%, preferably 0.1-30 w-%.